There has been known a surface-mount tuning-fork type crystal unit having an internal illumination and a cover made of a transparent material.
Also, there has been known a laser vibrometer which measures the vibration of a target object by laser light in real time using, for example, a polarization beam splitter. The laser vibrometer detects a vibration frequency of the target object by using the fact that linearly polarized light incident on the target object varies in frequency according to a vibration state of the target object.
In recent years, high-density mounting and miniaturization of components or modules is being progressed to meet the demand of the device miniaturization. There is no exception for a crystal unit serving as a clock source, and its miniaturization has been progressed. Under these circumstances, if a functional defect of the device is deemed to have occurred due to abnormality of the crystal unit, it would be useful if the electrical characteristics of the crystal unit may be measured in a mounted state. This is because, in the high-density mounting structure, peripheral parts may be destroyed when the crystal unit is removed, which makes it difficult to measure the characteristics after extracting only the crystal unit.
In order to measure the electrical characteristics of the crystal unit in the mounted state, it is necessary to incorporate a light source in the crystal unit, but the structure of the crystal unit is complicated. Further, a conventional measurement method is suitable for measurement of a target object which vibrates in a direction of an optical axis of the linearly polarized light, but it is not suitable for measurement of the characteristics of the crystal unit in the mounted state. This is because it is difficult to make the linearly polarized light incident such that the optical axis of the linearly polarized light coincides with the oscillation direction of the crystal unit in the mounted state.
In this regard, in the mounted state of the crystal unit, high impedance probe measurement may be realized. However, along with the recent trend of miniaturization, an integrated circuit (IC) has no terminal allowing the oscillation state to be checked, and the crystal unit has a terminal on the back surface, so that there may be no probing point. Further, in the high-density mounting structure, there may be no location on which a probe is placed physically. In addition, even though the probing point exists, the oscillation state may be changed by simply adding pF capacitance having a slightly low value by using the probe, which may make accurate measurement impossible.
The followings are reference documents.    [Document 1] Japanese Laid-Open Patent Publication No. 2004-282141 and    [Document 2] Japanese Laid-Open Patent Publication No. 2007-285898.